This invention relates to the improvement of the compatibility of a lubricating oil composition comprising dispersants containing basic nitrogen atoms, with fluorocarbon elastomer seals.
Lubricating oil formulations, particularly for the automotive industry, make use of a large number of additives, each having its respective role. The most important additives include detergents and dispersants, which, as their names indicate, are used to guarantee engine cleanliness and to keep in suspension the impurities produced in particular by the attack of the metallic or other parts of engines by the lubricating oil formulation.
The most widely used dispersants today are products of the reaction of succinic anhydrides substituted in the alpha positon by an alkyl chain of the polyisobutylene type (PIBSA) with a polyamine, possibly post-treated with a derivative of boron, ethylene carbonate or other post-treatment reagents known in the specialized literature. Among the polyamines used, polyalkylene-amines are preferred, such as diethylene triamine (DETA), triethylene tetramine (TETA), tetraethylene pentamine (TEPA), pentaethylene hexamine (PEHA) and heavier poly-alkylene-amines (HPA). These polyamines react with the succinic anhydrides substituted by alkyl groups of the polyisobutylene type (PIBSA) to give, according to the molar ratio of these two reagents, mono succinimides, bissuccinimides, or mixtures thereof.
These reaction products, possibly post-treated, generally have a basic nitrogen content of from 5 to 50, as measured by the base number or BN, expressed as mg KOH/g sample. This enables them to protect the metallic parts of an engine while in service from corrosion by acidic components formed as a result of the oxidation of the lubricating oil or the fuel, while keeping these oxidation products dispersed in the lubricating oil to prevent their agglomerization and their deposition in the casing containing the lubricating oil formulation.
These dispersants are even more effective if their relative basic nitrogen content is high, that is, insofar as the number of nitrogen atoms of the polyamine is larger than the number of succinic anhydride groups substituted by polyisobutenyl.
However, the higher the basic nitrogen content of these dispersants, the more they favor the attack of the fluorocarbon elastomer seals used in modern engines, because basic nitrogen tends to react with the acidic hydrogen atoms of this type of seal, and this attack results in the formation of cracks in the seal and the loss of the other desired physical properties of the fluorocarbon material from which it is made.
To resolve this dilemma, it has been proposed, according to U.S. Pat. No. 5,326,552 to Chevron, to subject the dispersants of the mono- or bissuccinimide type to a post treatment by reaction with a cyclic carbonate. Such a process not only improves the sludge dispersion in a lubricating oil containing these additives, but also the compatibility of the oil with a fluorocarbon elastomer seal.
Another solution is the subject of patent application WO 93/07242, also filed by Chevron, wherein the compatibility of a lubricating oil comprising additives containing basic nitrogen atoms with fluorocarbon elastomer seals is guaranteed by the addition of borated aromatic polyols, such as borated alkyl catechols.
Furthermore, it is well known that, in order to meet the longevity requirements demanded today in internal combustion engines, the lubricating oil formulations must contain many other ingredients, each of which has a very specific role.
Accordingly, besides the dispersants of the preceding type, other detergents are added, such as sulphonates, alkylphenates or metallic salicylates, sulphurized or not, anti oxidants, particularly zinc dialkyl dithiophosphates, antiwear and extreme pressure agents, foam inhibitors, friction reducers, rust inhibitors, corrosion inhibitors, pour point depressants, viscosity index improvers and many other additives.